This application claims benefit of Japanese Application No. Hei 11-281032 filed in Japan on Oct. 1, 1999, the contents of which are incorporated by this reference.
The present invention relates to an image pickup optical system and, more particularly, to an image pickup optical system including a reflective decentered image pickup optical element and a diffractive optical element.
Japanese Patent Application Unexamined Publication (KOKAI) No. Hei 10-333040 is known as a conventional compact reflective decentered image pickup optical system capable of two-dimensional imaging. The known optical system has at least a rear optical unit on the image side of a pupil plane. When a light ray emanating from the center of an object and passing through the pupil center to reach the image center is defined as an axial principal ray, the rear optical unit has an optical system with at least three surfaces each decentered so that the whole surface is tilted with respect to the principal ray. The surfaces include a rotationally symmetric curved surface that has both reflecting and transmitting actions and a rotationally asymmetric curved surface that has a reflecting action and corrects rotationally asymmetric decentration aberrations caused by decentration.
Chromatic aberration produced in the conventional reflective decentered image pickup optical system cannot satisfactorily be corrected by the optical system alone. Consequently, the chromatic aberration causes the captured image to be degraded in image quality.
Conventionally, a coaxial optical system is corrected for chromatic aberration by combining a negative lens with a positive lens. With respect to a refractive positive lens, a negative lens can correct both chromatic aberration and curvature of field. With respect to a reflective positive lens, however, a negative lens tends to cause curvature of field to become unfavorably large, although it can correct chromatic aberration. The reason for this is that a reflective positive lens has positive curvature of field, whereas a refractive positive lens has negative curvature of field.
Meanwhile, a conventional reflective decentered ocular optical system has a relatively long focal length. Therefore, if an image is formed on a small-sized image pickup device, the image taking range becomes unfavorably narrow.
In view of the above-described problems with the prior art, an object of the present invention is to provide a compact image pickup optical system satisfactorily corrected for chromatic aberration as well as decentration aberrations and capable of providing a clear image with minimal distortion even at a wide field angle.
To attain the above-described object, the present invention provides an image pickup optical system including an image pickup optical element and a diffractive optical element, which are decentered with respect to each other. The image pickup optical element has at least three optical surfaces adjacent to each other. At least one of the three optical surfaces is formed from a curved surface. At least two reflections take place between the optical surfaces.
The reasons for adopting the above-described arrangement in the present invention, together with the function thereof, will be described below.
The image pickup optical system according to the present invention is characterized by including a reflective decentered image pickup optical element and a diffractive optical element placed on the object or image side of the image pickup optical element.
A diffractive optical element has very strong negative dispersion (Abbe""s number: xe2x88x923.45) and is therefore capable of correcting chromatic aberration produced by a positive lens. Moreover, because the Petzval sum is zero, the diffractive optical element has no effect on curvature of field. Therefore, only chromatic aberration can be further corrected without increasing curvature of field by combining a diffractive optical element with the reflective decentered image pickup optical element, which is a reflective positive lens.
Accordingly, by incorporating a diffractive optical element into a reflective decentered image pickup optical system with a wide image taking range, it is possible to correct chromatic aberration without increasing curvature of field and to provide a clear image with minimal distortion even at a wide field angle.
It is preferable to satisfy the following condition:
xe2x88x921 less than F less than 1xe2x80x83xe2x80x83(1)
wherein F is the value of (the focal length of the image pickup optical system) divided by (the focal length of the diffractive optical element).
It should be noted that the focal length of a decentered optical system is defined as follows. As shown in FIG. 9, when the direction of decentration of the optical system is taken in the Y-axis direction, a light ray which is parallel to an axial principal ray 2 and which has a small height d in the YZ-plane is made to enter the optical system from the object side thereof. The sine of the angle that is formed between the two rays exiting from the optical system in the YZ-plane is denoted by NAxe2x80x2yi, and NAxe2x80x2yi/d is defined as the power Py in the Y-axis direction of the entire optical system. Similarly, a light ray which is parallel to the axial principal ray 2 and which has a small height d in the XZ-plane is made to enter the optical system from the object side thereof. The sine of the angle that is formed between the two rays exiting from the optical system in a plane perpendicularly intersecting the YZ-plane and containing the exiting axial principal ray is denoted by NAxe2x80x2xi, and NAxe2x80x2xi/d is defined as the power Px in the X-axis direction of the entire optical system. Furthermore, the reciprocals of the powers Px and Py are defined as the focal lengths Fx and Fy in the X- and Y-axis directions, respectively, of the entire optical system. In the present invention, the term (the focal length of the image pickup optical system) includes both the focal lengths Fx and Fy. In Examples (described later), however, the term (the focal length of the image pickup optical system) means the focal length Fy.
The condition (1) determines the aberration correction balance between the entire optical system and the diffractive optical element. If F is not smaller than the upper limit of the condition (1), i.e. 1, the amount of aberration corrected by the diffractive optical element becomes excessive. If F is not :larger than the lower limit, i.e. xe2x88x921, the amount of aberration corrected by the diffractive optical element becomes deficient. In either case, the balance of aberration corrected by the diffractive optical element with respect to chromatic aberration produced in the optical system is destroyed, and it becomes impossible to attain favorable aberration correction.
It is even more desirable to satisfy the following condition:
xe2x80x83xe2x88x920.1 less than F less than 0.1xe2x80x83xe2x80x83(1-1)
It is still more desirable to satisfy the following condition:
0 less than F less than 0.1xe2x80x83xe2x80x83(1-2)
It is practically preferable that the image pickup optical element in the image pickup optical system according to the present invention should be formed from a prism member in which the space defined by the at least three surfaces is filled with a :transparent medium having a refractive index larger than 1.
Preferably, at least one curved surface of the image pickup optical element is a rotationally asymmetric surface with no axis of rotational symmetry in the surface nor out of the surface. The rotationally asymmetric surface has a totally reflecting action or a reflecting action. When a light ray emanating from the center of an object and passing through the pupil center to reach the image center is defined as an axial principal ray, the rotationally asymmetric surface is tilted with respect to the axial principal ray. The rotationally asymmetric surface corrects rotationally asymmetric aberrations due to decentration by the rotationally asymmetric surface configuration. The diffractive optical element is placed on the object or image side of the image pickup optical element. With the above-described arrangement, it is possible to attain an image pickup optical system favorably corrected for both decentration aberrations and chromatic aberrations.
The rotationally asymmetric surface used in the present invention should preferably be a plane-symmetry free-form surface having only one plane of symmetry. Free-form surfaces used in the present invention are defined by the following equation (a). It should be noted that the Z-axis of the defining equation is the axis of a free-form surface.                     Z        =                                            cr              2                        /                          [                              1                +                                                      {                                          1                      -                                                                        (                                                      1                            +                            k                                                    )                                                ⁢                                                  c                          2                                                ⁢                                                  r                          2                                                                                      }                                                              ]                                +                                    ∑                              j                =                2                            66                        ⁢                                          C                j                            ⁢                              X                m                            ⁢                              Y                n                                                                        (        a        )            
In Eq. (a), the first term is a spherical surface term, and the second term is a free-form surface term.
In the spherical surface term:
c: the curvature at the vertex
k: a conic constant
r={square root over ( )}(X2+Y2)
The free-form surface term is given by             ∑              j        =        2            66        ⁢                  C        j            ⁢              X        m            ⁢              Y        n              =                    C        2            ⁢      X        +                  C        3            ⁢      Y        +                  C        4            ⁢              X        2              +                  C        5            ⁢      XY        +                  C        6            ⁢              Y        2              +                  C        7            ⁢              X        3              +                  C        8            ⁢              X        2            ⁢      Y        +                  C        9            ⁢              XY        2              +                  C        10            ⁢              Y        3              +                  C        11            ⁢              X        4              +                  C        12            ⁢              X        3            ⁢      Y        +                  C        13            ⁢              X        2            ⁢              Y        2              +                  C        14            ⁢              XY        3              +                  C        15            ⁢              Y        4              +                  C        16            ⁢              X        5              +                  C        17            ⁢              X        4            ⁢      Y        +                  C        18            ⁢              X        3            ⁢              Y        2              +                  C        19            ⁢              X        2            ⁢              Y        3              +                  C        20            ⁢              XY        4              +                  C        21            ⁢              Y        5              +                  C        22            ⁢              X        6              +                  C        23            ⁢              X        5            ⁢      Y        +                  C        24            ⁢              X        4            ⁢              Y        2              +                  C        25            ⁢              X        3            ⁢              Y        3              +                  C        26            ⁢              X        2            ⁢              Y        4              +                  C        27            ⁢              XY        5              +                  C        28            ⁢              Y        6              +                  C        29            ⁢              X        7              +                  C        30            ⁢              X        6            ⁢      Y        +                  C        31            ⁢              X        5            ⁢              Y        2              +                  C        32            ⁢              X        4            ⁢              Y        3              +                  C        33            ⁢              X        3            ⁢              Y        4              +                  C        34            ⁢              X        2            ⁢              Y        5              +                  C        35            ⁢              XY        6              +                  C        36            ⁢              Y        7            ⁢              xe2x80x83            ⁢      …      
where Cj (j is an integer of 2 or higher) are coefficients.
In general, the above-described free-form surface does not have planes of symmetry in both the XZ- and YZ-planes. In the present invention, however, a free-form surface having only one plane of symmetry parallel to the YZ-plane is obtained by making all terms of odd-numbered degrees with respect to X zero. For example, in the above defining equation (a), the coefficients of the terms C2, C5, C7, C9, C12, C14, C16, C18, C20, C23, C25, C27, C29, C31, C33, C35, . . . are set equal to zero. By doing so, it is possible to obtain a free-form surface having only one plane of symmetry parallel to the YZ-plane.
A free-form surface having only one plane of symmetry parallel to the XZ-plane is obtained by making all terms of odd-numbered degrees with respect to Y zero. For example, in the above defining equation (a), the coefficients of the terms C3, C5, C8, C10, C12, C14C17, C19, C21, C23, C25, C27, C30, C32, C34, C36, . . . are set equal to zero. By doing so, it is possible to obtain a free-form surface having only one plane of symmetry parallel to the XZ-plane.
Furthermore, the direction of decentration is determined in correspondence to either of the directions of the above-described planes of symmetry. For example, with respect to the plane of symmetry parallel to the YZ-plane, the direction of decentration of the optical system is determined to be the Y-axis direction. With respect to the plane of symmetry parallel to the XZ-plane, the direction of decentration of the: optical system is determined to be the X-axis direction. By doing so, rotationally asymmetric aberrations caused by decentration can be corrected effectively, and at the same time, productivity can be improved.
It should be noted that the above defining equation (a) is shown as merely an example as stated above, and that the feature of the present invention resides in that rotationally asymmetric aberrations caused by decentration are corrected and, at the same time, productivity is improved by using a rotationally asymmetric surface having only one plane of symmetry. Therefore, the same advantageous effect can be obtained for any other defining equation that expresses such a rotationally asymmetric surface.
Incidentally, the above-described prism member may be arranged as follows. The prism member has three optical surfaces: a first surface having a transmitting action; a second surface having both reflecting and transmitting actions; and a third surface having a reflecting action. Light from an object enters the prism member through the first surface. The light is reflected by the second surface and further reflected by the third surface and exits from the prism member through the second surface.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.
The invention accordingly comprises the features of construction, combinations of elements, and arrangement of parts which will be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims. dr
FIG. 1 is a sectional view of an image pickup optical system according to Example 1 of the present invention taken in the direction of decentration.
FIG. 2 is a sectional view of an image pickup optical system according to Example 2 of the present invention taken in the direction of decentration.
FIG. 3 is a sectional view of an image pickup optical system according to Example 3 of the present invention taken in the direction of decentration.
FIG. 4 is a sectional view of an image pickup optical system according to Example 4 of the present invention taken in the direction of decentration.
FIG. 5 is a sectional view of an image pickup optical system according to Example 5 of the present invention taken in the direction of decentration.
FIG. 6 is an aberrational diagram showing lateral aberrations in Example 1.
FIG. 7 an aberrational diagram showing distortion in Example 1.
FIG. 8 is a conceptual view showing an arrangement in which an image pickup optical system according to the present invention is incorporated into an image pickup apparatus.
FIG. 9 is a diagram for describing the focal length of the image pickup optical system according to the present invention.